Conventional thin-layer field-effect transistors (TFTs) utilizing silicon semiconductor or compound semiconductor are used in general integrated circuits and other expanding applications. While the number of new applications to which such devices are desirably applied is increasing, the requirements of lower cost and flexibility are imposed on these devices. To comply with such requirements, active research works have been made on organic semiconductors because of possible fabrication of devices having a variety of functions including low cost and flexibility. The implementation of organic semiconductors into a commercial practice will lead to the appearance of printable integrated circuits, electronic paper and the like. However, most organic semiconductors exhibit p-type behavior while only C60 and few other materials exhibit n-type behavior. N-type organic semiconductors are key materials for the establishment of organic electronic devices including p-n junctions.
In general, organic semiconductors are not prone to polarity inversion from p-type to n-type, because they have so great a band gap as compared with silicon semiconductor that no inversion layer is formed even when the band is deflected by applying an extra gate voltage. Although the inversion layer may be formed by inducing numerous carriers at the interface between the gate insulating film and the organic semiconductor, a high gate voltage can cause breakdown if a prior art gate insulating film is used. It is thus difficult to induce a sufficient quantity of carriers to provoke polarity inversion.
For instance, Appl. Phys. Lett., Vol. 85, p 3899 (2004) describes to use an aluminum oxide thin film having a high withstand voltage, high dielectric constant and low leakage current as the insulating film and a single crystal as the organic semiconductor. Allegedly the use of a single crystal as the organic semiconductor eliminates the influence of grains and trap level in the semiconductor thin film, and it is thus expected to gain a high mobility. Although the oxide insulator has the advantages of ease of thin film formation and a high dielectric constant, it undesirably has a low withstand voltage due to the essential presence of oxygen vacancies.